The invention relates to a method for processing a signal which is used to transmit data in the frequency domain by use of data symbols which are separated from one another by a respective guard interval and are each composed of a particular number of carriers. Such signals are known, in particular, from digital terrestrial television transmission (i.e. digital video broadcasting).
Such multi-carrier methods can be used to transmit data distributed over a plurality of carriers. In the case of parallel transmissions, the entire frequency band of a transmission channel is split into a plurality of subchannels having a correspondingly smaller frequency band. Each subchannel is modulated with a dedicated data sequence.
A subchannel contains a carrier onto which the actual information is modulated, as in the case of conventional transmission methods. During parallel transmission, a plurality of subcarriers are therefore present. In the case of orthogonal signals, the subchannels can be isolated using correlation methods. This allows interference disturbances between the individual channels to be eliminated.
A standardized transmission method that uses parallel data and frequency multiplexing is orthogonal frequency division multiplexing (OFDM). In OFDM, each carrier is orthogonal with respect to the other carriers. In the case of digital data transmission, and hence also in the case of OFDM, the data is transmitted not as a homogeneous data stream, but rather in a particular configuration called a frame. By way of example, each frame contains 68 so-called data symbols. Each data symbol in turn contains a set of a plurality of carriers that are transmitted in parallel. The individual data symbols are separated from one another by guard intervals. These serve to minimize disturbances caused by the data symbols overlapping one another. This is necessary to obtain good separation of the subcarriers using Fourier transformation. The guard intervals are obtained by cyclical continuation of the respective signals of the subcarriers in the time domain. In this context, the guard interval makes up a fraction of the duration of the data symbol, for example a quarter. The cyclical continuation of the data symbol by the guard interval allows carrier synchronization to be maintained in the receiver. Special reference carriers disposed at defined points within a data symbol are also used for synchronization purposes. The number of these reference carriers is less than the number of data carriers, which contain the actual information.
Besides pilot carriers, parameter carriers are also part of the reference carriers. The pilot carriers are used to transmit particular data sequences whose value is known at the reception end. The pilot carriers, whose position is defined within the frame, can be used to synchronize the receiver with the received signal.
The parameter carriers transmit to the receiver the parameters used during transmission, such as the modulation scheme and the length of the guard interval.
For OFDM, the number of data items and reference carriers and also further details are defined in European Telecommunication Standards ETS 300 744 from the European Telecommunications Standards Institute, Mar. 1997.
During transmission of a signal containing data symbols separated from one another by guard intervals, time periods with the duration of the guard intervals arise in which no data carriers are received. Gaps are therefore produced in the flow of data, the size of these gaps depending on the length of the guard intervals. Since a continuous flow of data is usually required at the output of the receiver, the data is read into a buffer store. The data buffer-stored in this manner is then read out again continuously using a fixed spacing. Examples of the buffer stores used are first-in first-out (FIFO) memories. Such a buffer store makes a reception module larger and more expensive.
It is accordingly an object of the invention to provide a method for processing a signal containing data symbols, that overcomes the above-mentioned disadvantages of the prior art methods of this general type, in which a continuous data stream is achieved at the reception end without the need for buffer storage.
With the foregoing and other objects in view there is provided, in accordance with the invention, a method for processing signals used to transmit data in a frequency domain. The method includes receiving a signal having data symbols with a symbol duration TU and guard intervals with an interval duration TD separating the data symbols from one another, each of the data symbols further having a particular number of carriers. The carriers are recovered from one of the data symbols of the signal in the frequency domain in a separator stage of a receiver. Then, at least a section of the carriers are used to determine the data transmitted with the data symbol such that the data has approximately a same time spacing over a total duration TS. The total duration TS is formed of the symbol duration TU and the interval duration TD.
The invention has the advantage that the carriers recovered by the separator stage do not need to be buffer-stored. A corresponding buffer store can therefore be dispensed with. In addition, further signal processing is simplified, because the carriers to be processed are present in a continuous train.
Evaluation can take place at a lower speed since the carriers transmitted over the transmission duration of a data symbol have at their disposal the transmission duration for the data symbol together with a guard interval. The invention is therefore advantageously used in conjunction with further methods for signal processing.
Signal processing includes, in particular, the detection of reference carriers, the detection of interference and channel estimation, which is used for the arithmetical correction of the transfer functions of the channel. In this context, differences between an actual transmission channel and a standard channel defined in terms of its transmission properties are determined. The transfer function is estimated by interpolation using a plurality of reference carriers.
In order to be able to carry out estimations at a particular instant, reference carriers are required which are not received until a later instant, that is to say are not yet available at the particular instant. This problem is solved by moving the estimation for the earlier instant to the later instant. The channel estimation for a particular instant is therefore delayed. The reference carrier information already available at the earlier instant needs to be stored on account of the delayed estimation.
The estimated transfer function of the channel is used to correct the data carriers. The phase and amplitude of the data carriers are altered such that they match the phase and amplitude of a corresponding data carrier that has been transmitted over the standard transmission channel.
Signal processing also includes detection of interference that may arise, for example, when analog signals are transmitted over the same channel.
Expediently, only those carriers in a data symbol that need to be used for signal processing are stored.
In accordance with an added feature of the invention, the section of the carriers is supplied to a signal processing stage using buffer-stored carriers.
In accordance with an additional feature of the invention, channel estimation is carried out in the signal processing stage.
In accordance with another feature of the invention, a function of the carriers is buffer-stored in different groups, with the carriers which were read into a respective group during an earlier storage operation being overwritten.
In accordance with a further added feature of the invention, there is the step of storing only the carriers needed for synchronization during synchronization of the receiver with the signal.
In accordance with a concomitant feature of the invention, the data are determined using a digital timing oscillator.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a method for processing a signal containing data symbols, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.